The principal objective of the proposed research is to elucidate the mechanisms of vascular smooth muscle relaxation by clinically employed vasodilator drugs and endogenous vasoactive substances. The former class includes organic nitrate and nitrite esters and sodium nitroprusside, and the latter class includes acetylcholine, arachidonic acid, certain autacoids, and peptides. Studies proposed here are focused to develop a better understanding of the pharmacological and physiological role of cyclic GMP in vascular smooth muscle relaxation. Many of the studies are designed to ascertain the influence of modulation of cyclic GMP metabolism on vascular smooth muscle tone through the use of pharmacological probes developed during the conduct of studies in the original proposal. The specific aims of the proposal are: 1) to determine the effects on arterial and venous tone of the modulation of resting levels of cyclic GMP by guanylate cyclase and cyclic GMP-phosphodiesterase inhibitors in the presence of intact and damaged endothelium; 2) to elucidate the mechanism of endothelium-dependent arterial relaxation elicited by acetylcholine, arachidonic acid, and autacoids, and to determine why veins generally contract under the same conditions; 3) to obtain solid experimental evidence for the formation and actions of arterial endothelium-dependent relaxant factors, to characterize their properties, and to study their effects on veins; 4) to elucidate the precise mechanism by which norepinephrine relaxes coronary artery in an endothelium-dependent but propranolol-insensitive manner; 5) to elucidate the mechanism of vascular smooth muscle relaxation elicited by atrial natriuretic factors; 6) to continue ongoing in vitro and in vivo studies with novel vasodilator S-nitrosothiols, with particular emphasis on determining the mechanism of selective tolerance to organic nitrate esters. The proposed research represents a more in-depth continuation of our observations that cyclic GMP appears to play an important role in vascular smooth muscle relaxant responses not only to nitrogen oxide-containing vasodilator drugs but also to endogenous vasoactive agents. New information from the proposed studies should provide us with more insight into mechanisms by which the endothelium modulates vascular smooth muscle tone, and how established and novel vasodilator drugs may be better employed clinically to treat disorders.